This invention relates generally to electrical switches and more particularly to manually operable trigger switches for use in hand-held power tools and the like. The invention also relates to power tools equipped with such switches.
Electrically driven hand-held power tools, such as portable drills, saws and sanders, are suited to a wide variety of applications and permit substantial savings of both time and physical effort required to perform a variety of tasks. While the overall safety record in the use of such tools has been and continues to be excellent, further effort to improve this record is nevertheless worthwhile.
Despite all reasonable precaution, it sometimes happens that hand-held power tools pose a risk of injury to their users. One such risk is the risk of electrical shock which, however small, is always present around devices of an electrically driven nature. Another such risk is the risk of reverse torque being applied to the hands of an operator as when, for example, a drill bit or saw blade binds in a workpiece during boring or sawing operations.
To minimize the chances of injury in the event a tool binds in a workpiece, the desirability of immediately stopping the tool upon the occurrence of such binding has been recognized. In one arrangement, a portable electric drill was provided in which a handle was pivotally mounted to the drill housing as to allow pivotal movement between the housing and the handle over a limited range. An electrical switch, responsive to such pivotal movement formed part of the motor drive circuit. If a drill bit became stuck in the workpiece, the resulting reverse torque caused the handle to pivot relative to the housing, causing the switch to open and the tool to stop. While the arrangement was effective, it had the disadvantage of being difficult to retrofit into existing units.
The present invention is based on the observation that when a difficulty, such as an inadvertent electric shock or unanticipated binding, is encountered in using a hand-held power tool, the normal reaction of the user is to grip the tool evermore tightly. When the tool is equipped with a conventional trigger switch, the result is that power continues to be supplied to the tool thereby increasing the possibility of injury or damage.
The present invention utilizes the user's normal instinctive reflex reaction when trouble is encountered, to automatically and quickly deenergize the power tool. This is accomplished by providing a trigger switch which, when subjected to normal finger pressure, energizes the tool, but which deactivates the tool when subjected to increased finger pressure. Thus, when difficulty is encountered and the operator's grip is tightened, the tool is automatically deactivated to avoid or minimize injury or damage. The deactivation can include not only an interruption of the flow of current to the tool's motor, but also activation of a braking mechanism for internally absorbing the energy of reverse torque. The braking function can be optional depending on the characteristics of the tool involved and the degree of sophistication of its design. Since the invention can be incorporated in replacement trigger switch assemblies, it may easily be retrofitted into existing power tools having conventional trigger switches.
In view of the foregoing, it is a general object of the present invention to provide an assembly for automatically deactivating power tools in the event an unanticipated, potentially dangerous situation is encountered.
It is another object of the present invention to provide an assembly for automatically deactivating hand-held power tools in the event difficulty is encountered which may easily be retrofitted to existing power tools having conventional trigger switches.